denying certain system users from checking for mail

Is there a way with popa3d to do like a deny.users list or
something?  I'm just looking for a way to tighten down
the system, to keep the script kiddies and such from
even getting to an authentication stage in the mail check,
for example when they use the news user, root user, etc.

I want the username to have to be a valid user of the
system, i.e. mail-retrieving user, before it will get
to the authentication stage.

TLD